Measurement of major organic acids in rainwater in Southeast Asia during burning and non-burning periods

Rainwater samples were collected in Singapore from August to December 1997 using a wet-only collector. Major organic acids were analyzed in all collected samples, using Ion Chromatography, with concentrations ranging from 1.4 to 105.6 mu eq/l (mean = 17.6 mu eq/l) for formate and from 1.7 to 62.8 mu eq/l (mean = 16.7 mu eq/l) for acetate, respectively. Elevated concentrations of both formate and acetate were observed during the months of September and October when the biomass burning impacted air masses originating from Indonesia had arrived at Singapore and prevailed there. The formate to acetate ratio was greater than 1 in biomass burning plumes suggesting that formic acid was likely produced in the atmosphere in addition to its direct emission from fires. Both formate and acetate together accounted for as much as 68 % of unneutralized total acidity in rain during the burning period. Our results suggest that biomass burning is an important source of HCOOH and CH3COOH to the troposphere over Southeast Asia.